Greetings and Salutations.
If you are seeing constant traffic (flashing data lights on the Cable Modem), it is likely that one of two things has happened.
1) It is possible that your system has gotten infected with a Windows Virus of some sort. I would update your Windows Virus Checker programs, then, reboot into safe mode, and run a complete scan of the system.
2) it is possible that you are simply the focus of a continuing attempt to break into your system by OTHER Windows Virus infected systems. You can guard about this by either making sure that the Windows Firewall is set properly, or, by downloading and installing any of a number of third party firewalls. These alternative programs are often easier to use and somewhat more resistant to being broken by external attacks.
If you are not fairly technically oriented, you probably should track down someone who has experience in running various of the network scanners and analysis programs to see what the traffic on the Network is, and to figure out where it is going, etc.
One small thing you can do to help is to go into the Network connections icon in the Control panel. On that control is a check box that will show you connection traffic as well as whether or not you are connected. Make sure that all the boxes are checked, then, shut down the control panel. You may have to reboot the computer. However, when this is done correctly, you should see a small icon down on the task bar (right end), that looks like two computer monitors. As network traffic moves in and out of your computer, the screens on the two monitors will flicker. Why is this useful, I am sure you are asking. Well, in theory, you should see almost no flickers unless you are actively surfing the Net. You will see a few because you probably have a number of programs running in the background that will phone home to see if there are any updates to be installed. Windows, RealPlayer, Flash, etc, all do this. If you are seeing a lot of traffic when you are just sitting at the desktop, this is a good hint that something is doing WAY too much talking on the Net...and might well be a Windows Virus or other Windows Malware.
By the by...It may bring screams of outrage here, but, the best person to track down these issues MAY not be your 10 year old nephew...and possibly not the Geek Squad. I would suggest finding a smallish computer store, or ask around to see if there are some consultants that can help you. Network issues can be tricky, and, even today, good analysis takes a combination of knowledge, experience and time...
Regards
Dave Mundt